Always Underleveled

Log off in a cantina for a week. You'll be getting bonus xps till the end of time.

Lol. Well thats not a viable solution for me.

Please don't take this the wrong way, but the single most likely answer is user error based on some kind of selective bias. In other words, you skip more than you think you do, but don't recall it that way.

I admit it's entirely possible you may be suffering from an unusual bug that has reduced your xp intake, but that seems far less probable than you just missing more bonus quests than you think.

Yeah, that is the first thing I thought about but the only thing I don't do is heroics. Unless there is some magical questing hub that I am missing on every planet.

Sure, but this is just a matter of mathematics. XP goes in, levels come out, at known rates. So either you're wrong, or you're bugged.

You might want to open a ticket to CS, and in the meantime, do WZs and FPs to catch up on the levels. The dailies for WZs and FPs are nice sources of extra XP, as are the matches and dungeons themselves.

If CS looks at your logs and sees a bug, they are unlikely to fix it retroactively, but they should be able to correct it going forward. If they don't see anything, then that increases the odds that you're simply in error (though it doesn't establish it conclusively, as EA's CS is not really anything to write home about) and you can consider trying to be more conscious about bonuses and the like.

You can pick up the nar shaddaa bonus series at level 28 in the spaceport.
The alderaan bonus series at level 40 from either house taral or organa, depending on your faction.
The hoth bonus series at level 45 from the home base on hoth.
By picking up these bonus series as soon as they become available, you maximize the xp you will get from them. Also, being under leveled may make the game a bit more challenging, but as long as you are properly geared and you're using your best companion that compliments your advanced class, then you should still be able to accomplish the quests.

I even skip the flashpoints and I'm still above level.
Now that I think of it, there are some flashpoints I have never done or done once.

The next time you do a class mission, record the xp you get. Then google the mission to make sure you get the right amount. I have a lvl 26 BH and skipped all heroics and space missions. I have done maybe five FPs on that toon. I skipped most of nar shardaa because I was four levels over the missions. Have moved on to tat and am now on level.

I m always overleveled for the quests im doing and it makes the game look really easy, i want less xp gain

tbh, if you make sure your gear and your companions gear is up to par for your lvl, then those 2 levels shouldn't make a difference, you'll be able to finish your class quest.

Did you complete Quesh? Many people just do the class quest and elave that planet for what it is. If you are on Hoth, I suggest exploring the planet, there are some hubs that have some more quests, plus, exploring gives good xp.